OFETs with active channels formed of densified layers

ABSTRACT

The present invention provides apparatus and a method of fabricating the apparatus. The apparatus includes a substrate having a surface and an organic field-effect transistor (OFET) located adjacent the surface of the substrate. The OFET comprising a gate, a channel, a source electrode, and a drain electrode. The channel comprises a densified layer of organic molecules with conjugated multiple bonds, axes of the organic molecules being oriented substantially normal to the surface.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed, in general, to a method ofmanufacturing organic field effect transistors (OFET) and, morespecifically, to methods of manufacturing OFETS having high mobilitychannels and the resulting OFET structure.

BACKGROUND OF THE INVENTION

There is an increasing interest in the use of organic field effecttransistors (OFET) where an active channel of the transistor is madefrom an organic material. To be useful, the organic material must becapable of supporting a channel of holes or electrons when the device isswitched on by applying a voltage to a gate associated with the channel.Moreover, the carrier or field effect mobility of the organic materialmust be sufficient to facilitate a large increase in conductivitybetween the source and drain when the device is on versus off.

The active channel in conventional OFETs is composed of organicmolecules, such as polymers or oligomers, having conjugated bonds. Forinstance, overlap between pi-orbitals of adjacent organic moleculesallows the channel to be conductive. In some cases, doping, by eitherremoving (via oxidation) or inserting (via reduction) electrons into theorganic molecule, can improve their conductivity. Approaches to improvethe carrier mobility of the channel include hydrogen bonding columnarstacks of organic molecules together and preparing alignment layers fororganic molecules on the substrate. Nevertheless, existing OFETscontinue to have channel regions with low carrier mobility and hencepoor sensitivity, due in part, because of the small difference in thedrain current in the “on” versus “off” state.

Accordingly the present invention overcomes the disadvantages associatedwith prior art devices by providing an OFET having a channel whosecarrier mobility is higher than previous OFETs and a method for thefabrication of such a device.

SUMMARY OF THE INVENTION

The present invention recognizes that the carrier mobility of thechannel of existing OFETs is low because the density and the uniformityof linear organic molecules packed in the channel are inadequate.Increasing the density and uniformity of the organic molecules in thechannel improves the carrier mobility of the channel. This, in turn,increases the conductivity of the channel and allows the manufacture ofOFET devices with improved sensitivity.

One embodiment of the present invention provides an apparatus. Theapparatus comprises a substrate having a surface and an organicfield-effect transistor located adjacent the surface of the substrate.The transistor comprises a gate, a channel, a source electrode, and adrain electrode. The channel includes a densified layer of organicmolecules with conjugated multiple bonds, axes of the organic moleculesbeing oriented substantially normal to the surface.

In another embodiment, the invention further provides a method of makingthe organic field-effect transistor. The method provides a substrate andforms a channel for an organic field-effect transistor. Forming thechannel includes expanding the substrate from an original configurationto increase a dimension of the substrate to an expanded dimension. Alayer of organic molecules with conjugated multiple bonds is formed onthe substrate while in the expanded dimension. The substrate is returnedto substantially the original configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood from the following detaileddescription, when read with the accompanying FIGUREs. Various featuresmay not be drawn to scale and may be arbitrarily increased or reducedfor clarity of discussion. Reference is now made to the followingdescriptions taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 schematically illustrates a detailed sectional view of anapparatus of the present invention;

FIGS. 2A to 2G schematically illustrate detailed sectional views of amethod of the present invention at selected stages of the method; and

FIGS. 3A to 3C schematically illustrate detailed sectional views of analternative method of the present invention at selected stages of thealternative method;

DETAILED DESCRIPTION

The present invention benefits from an understanding why the channelregion of existing organic field-effect transistors (OFETs) areinadequate. Specifically, the organic molecules of the channel have lowpacking density and are not uniformly packed. Consequently, theperformance of conventioanl OFETs suffer because the carrier mobility oftheir channels are poor. Moreover, prior to the present invention it wasnot recognized how the density or uniformity of packing of organicmolecules could be increased. This failure is due, in part, to the factthat the traditional substrates used in OFETs, such as silicon orvarious plastics, are inelastic.

The present invention recognizes for the first time that the use of anelastic substrate provides a novel result-effective variable forincreasing the density and uniformity of organic molecules that form thechannel region of an OFET. The term substrate as used herein refers toany elastic material capable of resuming its original shape when adeforming or expanding force is removed. Forming a layer of organicmolecules on the substrate while it is expanded allows a denser layer oforganic molecules to form than previously obtainable. Moreover, theorganic molecules are more densely packed without the need for hydrogenbonding between molecules to hold the molecules together. Thisadvantageously allows a broader class of organic compounds to be used toform the channel. In addition to the added challenges in synthesizingsuch materials, molecules containing polar groups, such as H-bondingsites, tend to absorb moisture more readily, thereby leading to morerapid device failure due to corrosion and shorting.

In addition, the compressive force associated with returning thesubstrate to its original configuration facilitates the alignment of theorganic molecules in one orientation. Furthermore, the more uniformorientation of the organic molecules can be achieved without the addedtechnical complications of preparing an alignment layer on the substratesurface. Conventional alignment layers typically involve a rubbedsurface with grooves and ridges, which can disrupt charge carriertransport in the direction perpendicular to the rubbing direction. Forinstance, a friction-transferred Teflon™ alignment layer has such a lowsurface energy that most solvents can not wet this surface properly,which in turn, leads to non-uniformity in film formation.

A denser and more uniformly packed layer of organic molecules isadvantageous because the charge transfer characteristics of the layer oforganic molecules depends on the density and uniform orientation of theorganic molecules, which generally each have one or more conjugatedbonds, to form a system of conjugated bonds. The efficiency of chargetransfer from one organic molecule to another increases as the distancebetween the conjugated bonding system of adjacent organic molecules isdecreased.

FIG. 1 illustrates a schematic representation of a portion of anexemplary apparatus 100 of the present invention. The apparatus 100 maybe used in any number of applications, such as biosensors, integratedcircuits, displays and memory devices. The apparatus includes asubstrate 105 having a surface 110, such as polydimethyl siloxane (PDMS)and an organic field-effect transistor 115 located adjacent the surface110 of the substrate 105. The transistor 115 comprises a gate 120, achannel 125, and source and drain electrodes 130, 135. The source anddrain electrodes 130, 135 are made of metal, such as gold, or conductiveorganic polymer. The gate 120 includes a gate electrode 140 also can bemade of gold, and a gate dielectric 145 composed of silicon or aluminumoxide. Other configurations of the OFET 115, well known to those ofordinary skill in the art, are within the scope of the presentinvention.

The channel 125 comprises a densified layer of organic molecules withconjugated multiple bonds 150, such as sexithiophene. Preferably, thelong axes of the organic molecules 155 are oriented substantially normal(e.g., about 90 degrees), to the substrate's surface 110. Of course, inother advantageous embodiments, a short axes 157 could be orientedsubstantially normal (e.g., about 90 degrees) to the substrate's surface110.

Herein, a layer of organic molecules 150 is referred to as densified ifthe layer 150 is physically strained parallel to the substrate-surface110 on which the layer 150 is formed. In a densified layer 150, thestrain is a force of expansion that results from an over-density oforganic molecules. A densified layer 150 has a strain, because themolecular over-density has not relaxed to a lower value that would befound in a bulk layer of the same composition. The densified layer 150is too thin for complete relaxation of the strain, which is caused bythe over-density of molecules at the substrate-surface. One way to forma densified layer of molecules 150 involves depositing the molecules ona stretched substrate 105 and then, allowing the substrate 105 tounstretch. Such a process can produce an over-density of molecules thatwould not have occurred if the layer had been formed directly on anunstretched substrate.

Preferably, the densified layer of organic molecules 150 has a surfacedensity of at least about 7 molecules/nm². It is also preferable thatadjacent organic molecules 160 of the densified layer 150 have anaverage separation distance 165 of less than about 3.8 Angstroms, andmore preferably, less than about 3.5 Angstroms. For adjacent organicmolecules 160 having conjugated pi bonds, the average separationdistance 165 refers to the distance between the pi bonds.

As noted above, it is advantageous for the organic molecules of thedensified layer 150 to have a uniform orientation because this improvescharge mobility. A uniform orientation is indicated by the polarizationratio of greater than about one. The polarization ratio is defined asthe ratio of transmitted light of one polarization to transmitted lightwith a perpendicular polarization. Polarization optical spectroscopyprocedures for measuring the polarization ratio are well known to thoseskilled in the art, and therefore need not be described here. See e.g.,Chen, X. L., Lovinger, A. J., et al., J., Chem. Mater. 13, 1341 (2001);Xue, G., et al., Langmuir 16, 1834 (2000); Amundson, K. R., et al. ThinSolid Film 414, 143 (2002); Sirringhaus H., et al., Appl. Phys. Lett.77, 406 (2002), which are incorporated by reference herein in theirentirety.

Preferably, the polarization ratio of the densified layer of organicmolecules 150 is greater than about one, and even more preferablygreater than about 2. Even more preferably, a direction of uniformorientation of the densified layer of organic molecules 150 issubstantially coincident in a direction of current flow 170 between thesource and drain electrodes 130, 135. In some embodiments, however, thecurrent flow 170 perpendicular to the expanded direction can also have ahigher charge carrier mobility, and therefore it is desirable to arrangethe drain and source electrodes 130, 135, accordingly.

Numerous alternative embodiments of the apparatus 100 are within thescope of the present invention. The substrate 105 is preferably anelastomer that can be expanded to at least three times its originallength and, once the exerted expanding force is released, the substrate105 returns to substantially (e.g., within about 10 percent) itsoriginal length. Examples of suitable elastomers include polyisoprene,polybutadiene, polychloroprene, poly(ethylacrylate),polybutylmethacrylate or silicon rubbers. Preferred silicon rubbersinclude alkyl-substituted polysiloxanes such as PDMS. Preferably, theelastomeric substrate 105 has a glass transition temperature (T_(G)) ofless than about 300° C. and more preferably, less than about 23° C.

In some advantageous embodiments of the apparatus 100, the organicmolecules of the densified layer 150 have a conjugated pi system ofbonds, although any semiconductive materials of organic molecules arewithin the scope of the present invention. Preferably, the organicmolecules of the densified layer of 150 have an ordered crystalline orpolycrystalline structure. The organic molecules can be any carboncontaining compounds capable of forming a semiconducting densified layer150. In some embodiments, the densified layer 150 is substantially freeof organic molecules having chemical groups capable of hydrogen-bondingbetween adjacent organic molecules 160.

Preferably, the densified layer 150 has a field effect mobility ofgreater than about 10⁻⁴ cm² V⁻¹ s⁻¹. Even more preferably, the densifiedlayer 150 has a field effect mobility of greater than about 10⁻² cm² V⁻¹s⁻¹, such as that provided by molecules having a conjugated pi system,as exemplified by poly- or oligothiophenes such as alpha-sexithiophene.In other advantageous embodiments, a ratio of the drain current of theOFET 115 when a voltage is applied to the gate 120, versus no appliedgate voltage, is at least about 10:1. For example, when the gate voltageof −100 V is applied for a p-channel organic semiconductor, theconductivity from source to drain 130, 135, in the densified layer 150,that is within about 50 Angstroms of an interface 172 between the gatedielectric 145 and densified layer 150, is at least about 10⁶ S cm⁻¹.

Preferred organic molecules of the densified layer 150 includepolythiophene and oligothiophene. For the purposes of the presentinvention, an oligothiophene has between 2 and 100 repeating units ofthiophene while polythiophene has greater than 100 repeating units ofthiophene. Other semiconductive organic compounds, however, are alsowithin the scope of the present invention. Nonlimiting examples includeoligophenyl or polyphenyl compounds. The organic molecule of thedensified layer 150 may also comprise combinations of different benzoidaromatic ring structures like benzene, napthalene or anthracene ringscoupled to each other such as pentacene, nonbenzoid aromatic rings,heterocylic rings, such as thiophene, or copolymers of these structures,such as copoly(bithiophenefluorene).

Some preferred organic molecules of the densified layer 150 havesubstantially coplanar aromatic groups, as this facilitates denser andmore uniform packing. The term substantially coplanar aromatic groupsrefers to at least two adjacent aromatic groups in an organic moleculehaving a twist angle of less than about 23 degrees between aromaticgroups. For example, adjacent thiophene rings in alpha-sexithiophenehave a twist angle of about 10 degrees, and therefore the thiophenerings are considered to be substantially coplanar. In contrast, the twofused benzene rings in a biphenyl structure have a twist angle of about23 degrees, and therefore the benzene rings are not coplanar.

In some preferred embodiments, the organic molecules of the densifiedlayer 150 are linear organic molecules because this also facilitatesdenser and more uniform packing. Examples of linear organic moleculesinclude polythiophenes, oligothiophenes such as alpha-sexithiophene, orderivatives of thiophenes, such as copoly(bithiophene fluorene). Inother embodiments, however, branched organic molecules or molecules withsidechains, such as oligothiophenes with alkyl chain substituted at the3-positions of thiophenes, can be used.

The densified layer 150 may comprise a single molecular layer of theorganic molecules or multiple layers of organic molecules. The densifiedlayer 150 can have a thickness 175 of about 20 Angstroms or more. Insome preferred embodiments, the densified layer 150 has a thickness 175between about 2 and about 100 Angstroms. In certain embodiments, athickness of less than about 5 Angstroms is preferred because thecarriers are confined mostly within a channel that is within 50Angstroms from the gate dielectric surface.

In some embodiments of the apparatus 100, it is preferable to covalentlybond the organic molecules of the densified layer 150 to the substratesurface 110. Covalent attachment to the substrate 105 facilitates theproduction of a denser, more uniform layer 150. In some instances, thesubstrate surface 110 is functionalized to promote covalent attachmentof organic molecules of the densified layer 150 to the substrate surface110, as further described below. In such embodiments, it is advantageousto also functionalize the organic molecules of the densified layer 150with trichlorosilanes or methyoxysilanes to facilitate covalent bondingto the stretched substrate. In other embodiments, however, the organicmolecules of the densified layer 150 are simply deposited on thesubstrate surface without covalent attachment to the substrate 105.

FIGS. 2A to 2G illustrate selected steps in an exemplary method of thepresent invention to form an OFET device 200. Turning first to FIG. 2A,the method includes providing a substrate 205 comprising, for example,PDMS or other materials as described above. Referring now to FIG. 2B,illustrated is the formation of source and drain electrodes 210, 215over the substrate 205. Metal source and drain electrodes 210, 215, canbe made of materials such as gold, silver, platinum, and palladium. Suchmaterials can be deposited by conventional techniques like vacuumdeposition, thermal evaporation or electron beam evaporation, followedby patterning via conventional lithographic processes to define theelectrode structures 210, 215. Alternatively, source and drainelectrodes 210, 215 can be made of conducting polymers, such aspolyanilines or polythiophene, that can be doped to increaseconductivity, or conductive ink, comprised of graphite and conductingpolymers. Conventional techniques, such as ink jet printing, screenprinting, or molding, can be used to form these source and drainelectrode structures 210, 215.

Referring now to FIG. 2C, the substrate 205 is expanded from itsoriginal configuration, such as shown in FIGS. 2A and 2B, to increase atleast one dimension of the substrate 205 to an expanded dimension 220.For instance, the substrate can be expanded by at least about 10percent, and more preferably, at least about 50 percent, as compared toits original configuration. The substrate 205 can be expanded by heatingat least a portion of the substrate 205. As well-understood by thoseskilled in the art, certain materials suitable for use as a substrate205 have large thermal expansion coefficients, and therefore, willexpand upon heating. Alternatively, the whole substrate 205 can beheated, in which case, all three dimensions of the substrate 205 can beexpanded simultaneously. Other methods can be used to expand thesubstrate. For example, certain substrates 205, such as PDMS, can besoaked in a solvent, such as hexane, to swell the substrate 205 involume by about 100 percent or more.

Alternatively, the substrate 205 can be expanded by mechanicallystretching a portion of the substrate 205 in the expanded dimension 220.Of course, the substrate 205 can be stretched in one, two or threedimensions. In some preferred embodiments, however, it is desirable tothe stretch the substrate in one dimension 220, that is lateral to thelong axes 155 of the organic molecules shown in FIG. 1. In someembodiments, for instance the substrate is stretched in a dimension 220that is substantially coincident with a direction of intended currentflow 225 between the source and drain electrodes 210, 215. Stretching insubstantially the same direction as the intended current flow 225advantageously facilitates the formation of a densified layer of organicmolecules, with closer pi-pi bonding stacking, for example, as furtherdiscussed below. Alternatively, in other preferred embodiments, it isdesirable to stretch the substrate lateral to the long axes 155 of theorganic molecules and perpendicular to the direction of intended currentflow 225 (arrow 227 coming out of the plane of the cross-sectiondepicted in FIG. 2C). Stretching in substantially the perpendiculardirection 227 as the intended current flow 225 is advantageous becauseof the more uniform orientation of the densified layer of organicmolecules, which in turn, provides a higher charge carrier mobility inthe current flow direction 225. If the substrate 205 has a TG of lessthan about 23° C., then stretching can be performed at room temperature.Alternatively, the substrate 205 can be heated to above its T_(G),before stretching.

Next, as illustrated in FIG. 2D, a surface of the substrate 230 ismodified to change the substrate's surface properties. Surfacemodification can advantageously promote the growth of the organicmolecules on the substrate surface 230, and in some cases, facilitatethe covalent bonding of organic molecules to the substrate 205, asdiscussed below. A modified surface can also promote the formation of auniform layer of organic molecules via epitaxial growth of aself-assembling layer of organic molecules. As an example of surfacemodification, a PDMS substrate surface 230 can be altered to havesilanol groups (—SiOH) 235, by exposure to an oxygen plasma, representedby arrows 240, for 1-2 seconds in a reactive ion etcher (Plama-Therm,Inc.) with a flow rate of 30 sccm and a pressure of 30 mTorr at 100 V.In other embodiments of the method, however, the substrate surface 230is not modified.

Turning now to FIG. 2E, illustrated is the formation of a layer oforganic molecules having conjugated multiple bonds 245 on the substrate205 while the substrate is in the expanded dimension 220. In theillustrated method, one end of the organic molecules 250 are covalentlybonded to the substrate 205. Covalent bonding advantageously promotesthe formation of a more densely and uniformly packed layer of organicmolecules 245.

Consider, for example, the above-described PDMS substrate 205 whosesurface 230 is modified with silanol groups 235. A trichlorosilane- andvinyl-terminated linker molecule 255, such as tetradecyl-1-enyltrichlorosilane, is covalently attached to the silanol groups 235. Thevinyl groups are then oxidized by chemically generated ozone tocarboxyl-groups. The carboxyl groups are then reacted withalcohol-terminated organic molecules 245 via an esterification reactionto covalently attach the organic molecules 245 to the substrate 205.

Of course, other methods, well known to those skilled in the art, can beused to covalently attach the organic molecules 245 to the substrate205. See e.g., Collet J., et al., Appl. Phys. Lett. 76, 1339 (2000),which is incorporated by reference herein in its entirety. In otherembodiments of the method, however, the organic molecules 245 are simplydeposited on the modified or unmodified substrate 205 without covalentattachment, via vapor phase or solution deposition or other conventionalprocesses. Although this approach has the advantage of requiring fewerprocessing steps than covalent attachment, in some case there is thepotential for cracks to form in the film when the stretched substrate205 is returned to its original configuration.

Referring now to FIG. 2F, illustrated is the device 200 after returningthe substrate 205 to substantially its original configuration to formthe channel region 260. Of course, one skilled in the art understandsthat the substrate 205 may not return to exactly the same dimensions ithad before surface modification or covalently attaching the organicmolecules 245 to the substrate's surface 230. For instance, the portionof the substrate 205 that is expanded in one or more dimension 220,after being relaxed, can vary from its original configuration by up toabout 10 percent.

In some preferred embodiments, it is advantageous to return thesubstrate 205 to substantially the original configuration by relaxingthe expanded dimension 220 in one direction 265. The compressive forceassociated with relaxing the substrate 205 in one direction 265 promotesa more densified or uniform organization of the organic molecules 245 inthe same direction 265 as the compressive force. Even more preferably,relaxation to the original configuration is done in a direction 265substantially coincident with a direction of current flow 225 betweenthe source and drain electrodes 210, 215, as this improves the carriermobility of the channel 260.

In certain preferred embodiments of the method, the OFET device 200 hasa channel region 260 with a surface density of organic molecules 245that is at least about 10 percent, and more preferably, at least about50 percent greater, than a surface density of organic molecules thatcould be formed on the same substrate 205 when in an originalnon-expanded configuration.

Turning now to FIG. 2G, illustrated is the OFET 200 after forming a gate270. The gate 270, comprises a gate electrode 275 and gate dielectric280. The gate dielectric 280 can be formed by depositing an inorganiclayer, such as silicon oxide or aluminum oxide thermally evaporated overthe substrate 205. Alternatively, the gate dielectric 280 can be formedby spin-coating insulating organic polymers or organic polymer/inorganiccomposites, or by chemical vapor deposition of monomer or organicpolymers, such as poly-para-Xylylenes like parylene, on the substrate205. As an example, polymers such as polyimide or polymethylmethacrylatecan be deposited alone or in combination with titanium nanoparticlesthat serve to increase the dielectric constant of the gate dielectric280. See e.g., U.S. patent application Ser. No. 10/700,651, by Howard E.Katz et al., filed Nov. 4, 2003, incorporated herein in its entirety.The gate electrode 275 can be comprised of the same materials and formedusing the similar processes as described above for the source and drainelectrodes 210, 215. Of course, the gate electrode 275 can be formedfrom different materials and by different processes, than those used toform the source and drain electrodes 210, 215.

Of course, alternative process flows of the method are apparent to thoseskilled in the art. In some embodiments, for instance, it is desirableto expand the channel region 260 before forming other components, suchas the source and drain electrodes 210, 215. This advantageously avoidsunnecessarily subjecting device components to stretching and relaxationforces that could deleteriously affect their structure or function.

FIGS. 3A to 3C illustrate selected steps in yet another method to forman OFET device 300 according to the principles of the present invention.Turning first to FIG. 3A, shown is a first substrate 305, upon which isformed a channel 310 of an OFET 300 that comprises a densified layer oforganic molecules 315 with conjugated multiple bonds. Any of theabove-discussed methods and organic molecules can be used to form thechannel 310.

Referring now to FIG. 3B, illustrated is a second substrate 320 uponwhich is formed a gate 325, a source electrode 330, and a drainelectrode 335 of the organic field-effect transistor 300. Any of theabove-discussed methods and materials can be used to form thesecomponents. In some embodiments, however, the second substrate 320preferably comprises an inelastic material, such as plastic or silicon.Such materials are advantageous because of their low cost, generalavailability and familiarity of use in semiconductor device fabrication.

Forming device components such as the gate 325 and source and drainelectrodes 330, 335 on a substrate 320 that is not expanded has a numberof benefits. Similar to that noted above for another embodiment, thesecomponents are not subjected stretching and relaxation forces that coulddeleteriously affect their structure or function. In addition, thefabrication of different device components on different substratesallows these fabrication steps to be performed in parallel instead ofsequentially, thereby allowing a faster manufacturing process.

Turning now to FIG. 3C, illustrated is the device 300 after positioningthe channel 310 between the source and drain electrode 330, 335 andproximate the gate 325 by coupling the first and second substrates 305,320 together. Any number of conventional techniques for laminating thefirst and second substrate 305, 320 can be used. See e.g., U.S. Pat. No.6,596,569, incorporated by reference herein in its entirety.

Although the present invention has been described in detail, those ofordinary skill in the art should understand that they can make variouschanges, substitutions and alterations herein without departing from thescope of the invention.

1-11. (canceled)
 12. A method comprising, providing a substrate; andforming a channel for an organic field-effect transistor, comprising:expanding said substrate from an original configuration to increase adimension of said substrate to an expanded dimension; forming a layer oforganic molecules with conjugated multiple bonds on said substrate whilein said expanded dimension; and returning said substrate tosubstantially said original configuration.
 13. The method of claim 12,further including forming a gate, a source electrode, and a drainelectrode of said organic field-effect transistor.
 14. The method ofclaim 12, wherein said substrate is a first substrate, and said methodfurther includes forming a gate, a source electrode, and a drainelectrode of said organic field-effect transistor on a second substrateand positioning said channel between said source and drain electrode andproximate said gate by coupling said first and second substratestogether.
 15. The method of claim 12, wherein said expanding comprisesheating at least a portion of said substrate.
 16. The method of claim12, wherein said expanding comprises mechanically stretching a portionof said substrate in a lateral dimension substantially coincident with adirection of current flow between said source and drain electrodes. 17.The method of claim 12, wherein said expanded dimension is at leastabout 10 percent longer than an equivalent portion of said substrate insaid original configuration.
 18. The method of claim 12, whereinreturning comprises relaxing said expanded dimension to substantiallysaid original configuration in a direction substantially coincident witha direction of current flow between said source and drain electrodes.19. The method of claim 12, wherein forming said layer of organicmolecules comprises covalently bonding an end of said organic moleculesto said substrate.
 20. The method of claim 12, wherein said channel hasa surface density of said organic molecules at least 10 percent greaterthan a surface density of said organic molecules formed on saidsubstrate in said original configuration.